“The Counter-Action (opposition thrust) is simply a coup d’arret which may be applied in any case against your opponent’s
final thrust. The problem is to
anticipate him on his own line of thrusting.
For that reason, it is practicable only when you are able to act with
greater rapidity and precision than your adversary. The counter-action exposes the fencer to the same danger as the coup d’arret, but is superior to it, in that, at worst, it realizes the
object of a parry and fulfills it, if it is directed at the spot where his
opponent intends to finish his action.”
--From The Art of the Foil, by Luigi Barbasetti
Imagine that your opponent wants to hit you at a certain
point on your body. So, draw an
imaginary line from your opponent’s bell guard all the way to the exact spot
on your body that is the target of the attack.
This is the path that your opponent’s blade will take when he lunges at
you. Now, before your opponent’s
attack arrives, counterattack along that same imaginary line and hit your
opponent. Because you are using the
same space as your adversary, this should deflect your opponent’s blade and
render his attack harmless.
Sound hard to do? It
is, and it’s very rarely done in competition fencing. Most of the time it’s done by accident—one fencer
attacks and the other happens to quickly counterattack along the exact same
line, causing the initial attack to run into the opponent’s bell guard. Some referees don't even know how to call this correctly, and will just throw out the touch or call the contact a beat or a parry. Actually trying to intentionally perform an opposition thrust in higher levels of competition is paramount to suicide, since the attacker could easily just disengage or change his line of attack without losing priority once he sees what you’re up to.